HOW WE HEAR Tahoma Jr. High 8 th Grade Science Maple Valley, WA

Many animals such as reptiles, amphibians, birds and mammals - such as humans - rely heavily on their sense of hearing for survival. The anatomy of their ears is very similar (due to genetic similarities). Let’s take a look at what happens to sound waves as they approach our ears and what happens to allow us to hear a variety of sounds from a twig snapping (danger) to a beautiful melody in music (relaxing… maybe…)

Sound waves are pulses of condensed or compressed air molecules (more dense) alternating with rarefied air (less dense). Let’s look at an animation of this (click link):

Those sound waves are collected and directed into the ear canal (a tube leading inside the head) by a flap of cartilage and skin called the pinna. We commonly call it the “ear”, but it’s really just a small part of a more complex system. However, it’s a great advantage for prey animals to be able to rotate the pinna – ever see a horse or rabbit do this? one forward and one back

The complex ear is divided into three main parts: Outer, Middle and Inner The pinna and ear canal make up the Outer Ear.

You can see how effective the pinna are by cupping your own hands to make a bigger pinna. Try this and listen to someone talk with and without large pinna – which can help your hearing by 2 to 3 times. Before hearing aids came along, people used “ear horns”.

Now that the compressed air waves have been focused into the ear canal, they will hit the eardrum (or tympanic membrane). Ever “hear” of a tympani drum?... get it… “hear of ”? here’s a frog’s tympanic membrane

The eardrum is a flexible membrane stretched across the opening and will vibrate when hit by the compressed air molecules. This area is the Middle Ear. One of the three smallest bones in the human body is attached on the back of the eardrum, (commonly called the “hammer”) and then attached to the other two bones (the “anvil” and “stirrup”. Doctors call them “ossicles”.

In this Middle Ear area, you need to be able to equalize pressure with the outside air, so there is a tube running to your mouth called the “Eustachian Tube”. If you open your mouth wide and wiggle your jaw, you can hear little clicks in your ears – these are the Eustachian Tubes opening. You may have noticed after yawning you hear better – the pressure inside and outside your head was equalized – letting the eardrum vibrate easier.

The three bones act together as an amplifying lever arrangement, and the stirrup attaches to the oval window, allowing vibrations to enter the cochlea. We’ll come back to the semi-circular canals later, they are not involved in hearing. But both the cochlea and semi-circular canals make up the Inner Ear.

The cochlea is filled with fluid and small hairs called “cilia”. As the fluid is vibrated, the hairs move – and they are attached to the auditory nerve that sends the signal of the sound to the brain for processing. High frequency sounds (like squeaks) don’t vibrate the hairs much. Low frequency (like rumbling thunder) move them a lot. Heavy bass music (the kind you feel more than hear) can quickly kill the cilia, and they don’t heal – causing permanent deafness.

Look at the scale above – CAREFULLY! – it is not linear (100, 200, 300, etc.). It is a “logarithmic” (or “log”) scale – multiplying by 10 each time! Although at first glance it looks like a cat or elephant may have the widest range of hearing, it’s actually the mouse. That’s because on right side of the scale, even a little “distance” of the line represents 10,000s of frequency. You have to look at where it starts and ends (about ,000 for mouse).

Now, about those semi-circular canals. You may have noticed there are three, arranged in three different directions: sideways, up and down, and on an angle. Each one is also filled with fluid and hairs, and lets you know what your orientation is in 3-D space (i.e. they give you your “balance”

If you spin yourself around, you spin the fluid outwards like the spin cycle on your washing machine. The hairs that are stimulated are not used to being stimulated this way, and your brain becomes confused – you become “dizzy”. If you get an ear infection, fluids from the sickness change the pressures in many areas of the ear and may cause dizziness. Tubes are sometimes put in the eardrum to allow draining the excess fluid.

Your “survival” as a student depends a lot on hearing – but not as much as you think. This is why it is CRITICAL that you not just pay attention in class, but go home and: read, write, speak, discuss and experiment (do) things on your own !!

If you’re not doing these things AND not even paying attention in class, you can forget about “hearing” your name being called at a graduation ceremony !! Don’t “dis” yourself. A little effort goes a LONG way !! End show